Systems mechanically driven mostly include gears or similar mechanical parts and an electric/electronic-driven device for driving them. Generally, if a pair of gears engage with each other, a gap with a certain space is required between their teeth where a driving gear (a gear generating a primary motion) and a driven gear (a gear generating a secondary motion according to the primary motion) engage with each other. The gap is used for introducing a lubricant between the gear teeth to reduce friction when both gears contact each other or for giving a space for absorbing thermal expansion caused by the material of the teeth.
If the pair of gears engaging with each other moves only in one direction, the gap does not cause a serious problem. However, if the gear pair moves in the opposite direction, the contact between the gear teeth may release so that the gears are instantly detached from each other and do not transfer power, thereby causing an error. Herein, the degree for a driven body, which has been moved to a certain location, to return to an original location is called a backlash.
In a mechanically driven system, the backlash is an important factor for enhancing accuracy of the driving. For example, a rotating plate is a combination of a gear, a linear actuator and an encoder or other combinations similar thereto, and mechanical errors of all, or a part of them influence the degree of the backlash. In addition, the backlash may be influenced by the performance of a motor; precision of gear processing; precision, temperature, humidity, abrasion or the like of a tool used for the gear processing; or the like. For example, in addition to the predictable error as described above, the backlash may accompany a processing error caused by gear fabrication due to a physical or technical limit, which is unpredictable. Even if the processing error is eliminated, the backlash could also be influenced by abrasion caused by long-time use, irregular thermal expansion caused by frictional feat, temperature changes, humidity, or the like.
Therefore, in order to enhance positional prediction in the gear system driven by a motor or the like, it is required to detect a backlash in an accurate and reproducible way. In particular, in industrial fields such as automated systems, precision control, robotics or the like, in which a high degree of precision is demanded, it is necessary to minimize even a small error in the driving system. This makes it important to detect a backlash precisely. In addition, in the technical fields demanding precise repeated works require a driving system to return to a starting position with reproducibility. Although backlash detecting techniques using torque, rotation angle or laser interferometer have been reported, at present the most precise backlash detection resolution is just in the unit of minute.